Motor vehicle powertrains typically incorporate a differential assembly which couples mechanical input power from a propeller shaft or other input member to drive a pair of vehicle wheels through axle halfshafts. Differentials allow differences in rotational speed to occur between the left and right-hand side driven axle halfshafts. The most basic design of differentials are known as open differentials and provide constant torque between the two axle halfshafts and do not operate to control the relative rotational speed between the axle shafts. A well-known disadvantage of open differentials occurs when one of the driven wheels engages the road surface with a low coefficient of friction (μ) with the other having a higher μ. In such operating conditions, the low tractive effort developed at the low p contact surface prevents significant toque from being developed on either axle. Since the torque between the two axles shafts is relatively constant for an open differential, little tractive effort can be developed to pull the vehicle from its position in the above described operating condition. Similar disadvantages occur in dynamic conditions when operating, especially in low μor so-called split μdriving conditions.
The above limitations of open differentials are well-known and numerous design approaches have been applied to address such shortcomings. One approach is known as a mechanical locking differential. These systems typically use mechanical or hydraulic actuators to couple the two axle halfshafts together such that they rotate at nearly constant speed. Thus in that operating condition, the two axles are not mutually torque limited. A mechanically based locking differential typically uses a clutch pack or interlocking components which lock the two axles together when a speed difference between the axles is detected or the operator commands that function. Other systems incorporate fluid couplings between the axles which provide a degree of mechanical coupling. Locking differentials which mechanically interlock the two axles together do not permit modulation of the coupling between the axles. Instead, the axles are either locked to rotate together or operate in the open condition. Other systems use electric motors or hydraulic pumps to actuate a coupling system across the differential. Electro-mechanical actuators are also used in some designs.
This invention is related to a differential actuator which enables a highly controlled coupling to occur between axle half shafts of a driven axle through a solenoid applied primary clutch. The differential assembly of this invention can be adapted for front, rear, or four-wheel drive applications. For example the differential assembly may be applied to a front wheel drive transaxle or an all wheel drive center differential.